Myelodysplastic syndromes (MDS) are stem-cell malignancies most frequently seen among elderly patients, resulting in a high annual incidence of approximately 14,000 new cases per year. The incidence of MDS continues to increase as our population ages. About 30-40% of MDS cases progress to acute myeloid leukemia (AML) with poor prognosis. The order of events occurring in the progression from normal health to MDS to acute leukemia is unknown. We have discovered a new oncogene, SALL4B, which is expressed constitutively in human leukemia cell lines and in almost 100% of primary AML cells. We have shown that transgenic mice that overexpress SALL4B exhibit MDS- like features and, subsequently, AML transformation associated with selective expansion of granulocyte/macrophage progenitor cells (GMPs) and hematopoetic stem cells (HSCs). The hypothesis underlying this proposal has two research components. First, we theorize that aberrant expression of SALL4B immortalizes HSCs/HPCs through activation of Bmi-1. Second, we theorize that SALL4B acts in concert with RAS and promotes progression of MDS to AML by enhancing proliferation of HSCs/HPCs. To test our hypothesis, we are proposing three specific but complementary aims designed to permit in-depth analysis of our SALL4B mouse model and to illuminate the development of MDS and AML. Specific Aim 1 focuses on determination of mechanisms leading to expansion of SALL4B-HSC/GMP associated with development of MDS and AML transformation. Specific Aim 2 focuses on identification of the molecular pathway(s) that mediates MDS progression. Specific Aim 3 focuses on cooperation of SALLB with second mutations in MDS progression. Our novel mouse model is ideal for investigating these diseases and gaining insight in the progression of MDS to fatal, late stage AML. The order and timing of oncogenic events involving MDS and MDS transformation are unknown and this is a major shortfall in our current understanding of these diseases. Our approach is innovative because it encompasses the role of a newly identified oncogene and a novel mouse model. Importantly, we expect to expand the body of knowledge surrounding MDS and MDS transformation consequently leading to the identification of novel therapeutic targets for treatment and intervention. Public Health Relevance Myelodysplastic syndrome (MDS) is, at present, incurable preleukemic disease occurring most frequently among the elderly with about 14,000 new cases per year in the USA. About 30-40% of these cases progress to acute myeloid leukemia (AML). With an urgent need to improve therapy in this group, it is critical to generate effective modeling systems (including mouse models) for investigating the genetic lesions commonly observed in MDS and AML. However, the precise molecular mechanisms whereby genetic changes induce the progression of MDS to AML remain poorly understood. The proposed research focuses on an innovative mouse model for studying MDS progression and AML transformation. We will also translate these studies into new diagnostic and prognostic tools for predicting those patients whose disease may progress to AML, which is most generally lethal to the patient.